Gestational Age by LMP: What the Date Assumes

LMP is the first day of your last menstrual period, and dating by it is a single subtraction: days since that day, divided by seven. Everything downstream — trimester, screening windows, due date — hangs off it.

One subtraction, three assumptions

The formula is trivial. What it hides is not.

By convention the estimated due date is 280 days after the first day of the LMP, a rule usually credited to Naegele. ACOG spells out what that rule quietly takes for granted: a regular 28-day cycle, with ovulation on the 14th day. Neither is checked before the date is written down, and neither is true for everyone.

So the method carries three failure points, all of them at the front:

1. Recall. ACOG cites published reports that about one half of women recall their last menstrual period accurately.
2. Cycle length. A 35-day cycle ovulates roughly a week later than a 28-day one, which makes the LMP-based age run about a week ahead of reality.
3. Ovulation timing. Even within a regular cycle, the day of ovulation moves.

None of these makes the LMP useless. It is the first estimate for every pregnancy, and it is free. It just is not the last word.

Worked example

Last period on March 1, today May 24:

• Days elapsed: 84
• Gestational age: 12 weeks and 0 days, since 84 divided by 7 is exactly 12
• Trimester: first, which runs through 13 weeks and 6 days
• Estimated due date: December 6, that is March 1 plus 280 days

How often the scan moves the date

This is the part worth knowing before your first appointment. ACOG describes a trial in which women were randomized to a first-trimester ultrasound: 40 percent of them had the due date adjusted because the scan and the last period disagreed by more than 5 days. In the group scanned only in the second trimester, just 10 percent were adjusted — not because the second group was better dated, but because a later scan is too blunt to catch the discrepancy.

Having a confident LMP does not exempt you from that. The same document notes that early ultrasound improves the accuracy of the due date even when the first day of the last period is known.

When the two do disagree, the cutoff for changing the date depends on how early the scan was; gestational age by ultrasound has the full table. For the overview of all three dating methods, see the gestational age calculator, and for the count expressed in plain weeks, pregnancy weeks calculator.

This content is informational and does not replace prenatal care. The date on your chart is set by a clinician.